Catalytic reduction of nox

ABSTRACT

A system for NO x  reduction in combustion gases, especially from diesel engines, incorporates an oxidation catalyst to convert at least a portion of NO to NO 2 , particulate filter, a source of reductant such as NH 3  and an SCR catalyst. Considerable improvements in NO x  conversion are observed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/843,870, filed Mar. 15, 2013, which was a continuation of U.S. patentapplication Ser. No. 13/204,634, filed Aug. 5, 2011, now U.S. Pat. No.8,480,986, which was a continuation of U.S. patent application Ser. No.12/380,414, filed Feb. 27, 2009, now U.S. Pat. No. 8,142,747, which wasa continuation of U.S. patent application Ser. No. 10/886,778, filedJul. 8, 2004, now U.S. Pat. No. 7,498,010, which was a divisionalapplication of U.S. patent application Ser. No. 09/601,694, filed Jan.9, 2001, now U.S. Pat. No. 6,805,849, which was the U.S. National Phaseof Int'l Pat. Appl. No. PCT/GB 1999/000292, filed Jan. 28, 1999, whichclaimed the benefit of priority from British Application No. 9802504.2,filed Feb. 6, 1998. These applications, in their entirety, areincorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention concerns improvements in selective catalyticreduction of NO_(x) in waste gas streams such as diesel engine exhaustsor other lean exhaust gases such as from gasoline direct injection(GDI).

BACKGROUND OF THE INVENTION

The technique named SCR (Selective Catalytic Reduction) is wellestablished for industrial plant combustion gases, and may be broadlydescribed as passing a hot exhaust gas over a catalyst in the presenceof a nitrogenous reductant, especially ammonia or urea. This iseffective to reduce the NO_(x) content of the exhaust gases by about20-25% at about 250° C., or possibly rather higher using a platinumcatalyst, although platinum catalysts tend to oxidize NH₃ to NO_(x)during higher temperature operation. We believe that SCR systems havebeen proposed for NO_(x) reduction for vehicle engine exhausts,especially large or heavy duty diesel engines, but this does requireon-board storage of such reductants, and is not believed to have metwith commercial acceptability at this time.

We believe that if there could be a significant improvement inperformance of SCR systems, they would find wider usage and may beintroduced into vehicular applications. It is an aim of the presentinvention to improve significantly the conversion of NO_(x) in a SCRsystem, and to improve the control of other pollutants using a SCRsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 1.

FIG. 2 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 2.

FIG. 3 is a graph plotting percentage NO_(x) conversion againsttemperature resulting from Test 3.

FIG. 4 is a bar graph showing percentage conversion of pollutants[NO_(x), particulates, hydrocarbons (HC) and carbon monoxide (CO)]resulting from Test 4.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the present invention provides an improved SCR catalystsystem, comprising in combination and in order, an oxidation catalysteffective to convert NO to NO₂, a particulate filter, a source ofreductant fluid and downstream of said source, an SCR catalyst.

The invention further provides an improved method of reducing NO_(x) ingas streams containing NO and particulates comprising passing such gasstream over an oxidation catalyst under conditions effective to convertat least a portion of NO in the gas stream to NO₂, removing at least aportion of said particulates, adding reductant fluid to the gas streamcontaining enhanced NO₂ to form a gas mixture, and passing the gasmixture over an SCR catalyst.

Although the present invention provides, at least in its preferredembodiments, the opportunity to reduce very significantly the NO_(x)emissions from the lean (high in oxygen) exhaust gases from diesel andsimilar engines, it is to be noted that the invention also permits verygood reductions in the levels of other regulated pollutants, especiallyhydrocarbons and particulates.

The invention is believed to have particular application to the exhaustsfrom heavy duty diesel engines, especially vehicle engines, e.g., truckor bus engines, but is not to be regarded as being limited thereto.Other applications might be LDD (light duty diesel), GDI, CNG(compressed natural gas) engines, ships or stationary sources. Forsimplicity, however, the majority of this description concerns suchvehicle engines.

We have surprisingly found that a “pre-oxidizing” step, which is notgenerally considered necessary because of the low content of CO andunburnt fuel in diesel exhausts, is particularly effective in increasingthe conversion of NO_(x) to N₂ by the SCR system. We also believe thatminimizing the levels of hydrocarbons in the gases may assist in theconversion of NO to NO₂. This may be achieved catalytically and/or byengine design or management. Desirably, the NO₂/NO ratio is adjustedaccording to the present invention to the most beneficial such ratio forthe particular SCR catalyst and CO and hydrocarbons are oxidized priorto the SCR catalyst. Thus, our preliminary results indicate that for atransition metal/zeolite SCR catalyst it is desirable to convert all NOto NO₂, whereas for a rare earth-based SCR catalyst, a high ratio isdesirable providing there is some NO, and for other transitionmetal-based catalysts gas mixtures are notably better than eithersubstantially only NO or NO₂. Even more surprisingly, the incorporationof a particulate filter permits still higher conversions of NO_(x).

The oxidation catalyst may be any suitable catalyst, and is generallyavailable to those skilled in art. For example, a Pt catalyst depositedupon a ceramic or metal through-flow honeycomb support is particularlysuitable. Suitable catalysts are, e.g., Pt/Al₂O₃ catalysts, containing1-150 g Pt/ft³ (0.035-5.3 g Pt/liter) catalyst volume depending on theNO₂/NO ratio required. Such catalysts may contain other componentsproviding there is a beneficial effect or at least no significantadverse effect.

The source of reductant fluid conveniently uses existing technology toinject fluid into the gas stream. For example, in the tests for thepresent invention, a mass controller was used to control supply ofcompressed NH₃, which was injected through an annular injector ringmounted in the exhaust pipe. The injector ring had a plurality ofinjection ports arranged around its periphery. A conventional dieselfuel injection system including pump and injector nozzle has been usedto inject urea by the present applicants. A stream of compressed air wasalso injected around the nozzle; this provided good mixing and cooling.

The reductant fluid is suitably NH₃, but other reductant fluidsincluding urea, ammonium carbamate and hydrocarbons including dieselfuel may also be considered. Diesel fuel is, of course, carried on boarda diesel-powered vehicle, but diesel fuel itself is a less selectivereductant than NH₃ and is presently not preferred.

Suitable SCR catalysts are available in the art and include Cu-based andvanadia-based catalysts. A preferred catalyst at present is aV₂O₅/WO₃/TiO₂ catalyst, supported on a honeycomb through-flow support.Although such a catalyst has shown good performance in the testsdescribed hereafter and is commercially available, we have found thatsustained high temperature operation can cause catalyst deactivation.Heavy duty diesel engines, which are almost exclusively turbocharged,can produce exhaust gases at greater than 500° C. under conditions ofhigh load and/or high speed, and such temperatures are sufficient tocause catalyst deactivation.

In one embodiment of the invention, therefore, cooling means is providedupstream of the SCR catalyst. Cooling means may suitably be activated bysensing high catalyst temperatures or by other, less direct, means, suchas determining conditions likely to lead to high catalyst temperatures.Suitable cooling means include water injection upstream of the SCRcatalyst, or air injection, for example utilizing the engineturbocharger to provide a stream of fresh intake air by-passing theengine. We have observed a loss of activity of the catalyst, however,using water injection, and air injection by modifying the turbochargerleads to higher space velocity over the catalyst which tends to reduceNO conversion. Preferably, the preferred SCR catalyst is maintained at atemperature from 160° C. to 450° C.

We believe that in its presently preferred embodiments, the presentinvention may depend upon an incomplete conversion of NO to NO₂.Desirably, therefore, the oxidation catalyst, or the oxidation catalysttogether with the particulate trap if used, yields a gas stream enteringthe SCR catalyst having a ratio of NO to NO₂ of from about 4:1 to about1:3 by volume, for the commercial vanadia-type catalyst. As mentionedabove, other SCR catalysts perform better with different NO/NO₂ ratios.We do not believe that it has previously been suggested to adjust theNO/NO₂ ratio in order to improve NO reduction.

The present invention incorporates a particulate trap downstream of theoxidation catalyst. We discovered that soot-type particulates may beremoved from a particulate trap by “combustion” at relatively lowtemperatures in the presence of NO₂. In effect, the incorporation ofsuch a particulate trap serves to clean the exhaust gas of particulateswithout causing accumulation, with resultant blockage or back-pressureproblems, whilst simultaneously reducing a proportion of the NOR.Suitable particulate traps are generally available, and are desirably ofthe type known as wall-flow filters, generally manufactured from aceramic, but other designs of particulate trap, including woven knittedor non-woven heat-resistant fabrics, may be used.

It may be desirable to incorporate a clean-up catalyst downstream of theSCR catalyst, to remove any NH₃ or derivatives thereof which could passthrough unreacted or as by-products. Suitable clean-up catalysts areavailable to the skilled person.

A particularly interesting possibility arising from the presentinvention has especial application to light duty diesel engines (car andutility vehicles) and permits a significant reduction in volume andweight of the exhaust gas after-treatment system, in a suitableengineered system.

EXAMPLES

Several tests have been carried out in making the present invention.These are described below, and are supported by results shown ingraphical form in the attached drawings.

A commercial 10 liter turbocharged heavy duty diesel engine on atest-bed was used for all the tests described herein.

Test 1—(Comparative)

A conventional SCR system using a commercial V₂O₅/WO₃/TiO₂ catalyst, wasadapted and fitted to the exhaust system of the engine. NH₃ was injectedupstream of the SCR catalyst at varying ratios. The NH₃ was suppliedfrom a cylinder of compressed gas and a conventional mass flowcontroller used to control the flow of NH₃ gas to an experimentalinjection ring. The injection ring was a 10 cm diameter annular ringprovided with 20 small injection ports arranged to inject gas in thedirection of the exhaust gas flow. NO_(x) conversions were determined byfitting a NO_(x) analyzer before and after the SCR catalyst and areplotted against exhaust gas temperature in FIG. 1. Temperatures werealtered by maintaining the engine speed constant and altering the torqueapplied.

A number of tests were run at different quantities of NH₃ injection,from 60% to 100% of theoretical, calculated at 1:1 NH₃/NO and 4:3NH₃/NO₂. It can readily be seen that at low temperatures, correspondingto light load, conversions are about 25%, and the highest conversionsrequire stoichiometric (100%) addition of NH₃ at catalyst temperaturesof from 325 to 400° C., and reach about 90%. However, we have determinedthat at greater than about 70% of stoichiometric NH₃ injection, NH₃slips through the SCR catalyst unreacted, and can cause furtherpollution problems.

Test 2 (Comparative)

The test rig was modified by inserting into the exhaust pipe upstream ofthe NH₃ injection, a commercial platinum oxidation catalyst of 10.5 inchdiameter and 6 inch length (26.67 cm diameter and 15.24 cm length)containing log Pt/ft³ (=0.35 g/liter) of catalyst volume. Identicaltests were run, and it was observed from the results plotted in FIG. 2,that even at 225° C., the conversion of NO_(x) has increased from 25%to >60%. The greatest conversions were in excess of 95%. No slippage ofNH₃ was observed in this test nor in the following test.

Test 3

The test rig was modified further, by inserting a particulate trapbefore the NH₃injection point, and the tests run again under the sameconditions at 100% NH₃ injection and a space velocity in the range40,000 to 70,000 hr⁻¹ over the SCR catalyst. The results are plotted andshown in FIG. 3. Surprisingly, there is a dramatic improvement in NO_(x)conversion, to above 90% at 225° C., and reaching 100% at 350° C.Additionally, of course, the particulates, which are the most visiblepollutant from diesel engines, are also controlled.

Test 4

An R49 test with 80% NH₃ injection was carried out over a V₂O₅/WO₃/TiO₂SCR catalyst. This gave 67% particulate, 89% HC and 87% NO_(x)conversion; the results are plotted in FIG. 4.

Additionally tests have been carried out with a different diesel engine,and the excellent results illustrated in Tests 3 and 4 above have beenconfirmed.

The results have been confirmed also for a non-vanadium SCR catalyst.

We claim:
 1. A method comprising: (a) passing an exhaust gas from adiesel engine over an oxidation catalyst to provide an adjusted gasstream, the exhaust gas comprising a first content level by volume ofNO, a first content level by volume of NO₂, and particulate matter, andthe adjusted gas stream comprising a second content level by volume ofNO that is lower than the first content level of NO, and a secondcontent level by volume of NO₂; (b) passing the adjusted gas streamthrough a particulate trap that results in trapping at least a portionof the particulate matter on the particulate trap; (c) combusting aportion of the trapped particulate matter such that there is nosignificant accumulation of particulate matter in the particulate trapin the presence of the adjusted gas stream at a combustion temperaturethat is lower than the temperature necessary to combust the trappedparticulate matter in the presence of the exhaust gas such that there isno significant accumulation of particulate matter in the particulatetrap, to create a further adjusted gas stream comprising a third contentlevel by volume of NO and a third content level by volume of NO₂ that islower than the second content level of NO₂; (d) injecting a reductantfluid comprising urea into the further adjusted gas stream; (e) mixingthe further adjusted gas stream with the injected reductant fluid toform a further adjusted gas stream mixed with reductant fluid; and (f)passing the further adjusted gas stream mixed with reductant fluid overan SCR catalyst to provide a final adjusted gas stream comprising afourth content level by volume of NO and a fourth content level byvolume of NO₂; wherein the second content level of NO₂ is sufficientlyhigher than the first content level of NO₂ such that when a portion ofthe second content level of NO₂ in the adjusted gas stream is consumedduring the combustion of the at least a portion of the trappedparticulate matter, the resulting third content level of NO₂ is stillsufficiently high for use with the SCR catalyst to provide the finaladjusted gas stream where the total combined volume of the fourthcontent level of NO and the fourth content level of NO₂ is lower thanthe total combined volume of the first content level of NO and the firstcontent level of NO₂, and the total combined volume of the fourthcontent level of NO with the fourth content level of NO₂is lowerrelative to the respective total combined volume of NO and NO₂ in afinal exhaust stream that would result from carrying out steps (b)-(f)starting with the exhaust gas instead of the adjusted gas stream.
 2. Themethod of claim 1, wherein the diesel engine is a vehicle engine.
 3. Themethod of claim 1, wherein the diesel engine is a heavy duty dieseltruck engine.
 4. The method of claim 1, wherein the diesel engine is aturbocharged heavy duty diesel truck engine.
 5. The method of claim 4,further comprising cooling the further adjusted gas stream.
 6. Themethod of claim 5, wherein the further adjusted gas stream is cooled byair supplied by the turbocharger.
 7. The method of claim 1, wherein theoxidation catalyst converts less than all of the NO in the exhaust gasto NO₂.
 8. The method of claim 1, wherein the further adjusted gasstream mixed with reductant fluid is at least 225° C. when passed overthe SCR catalyst, and the final adjusted gas stream has more than 90%less NO_(x) content by volume than the exhaust gas.
 9. The method ofclaim 8, wherein the final gas stream has at least 67% less particulatematter content by volume than the exhaust gas.
 10. A method comprising:(a) passing an exhaust gas from a diesel engine over an oxidationcatalyst to provide an adjusted gas stream, the exhaust gas comprising afirst content level by volume of NO, a first content level by volume ofNO₂, and particulate matter, and the adjusted gas stream comprising asecond content level by volume of NO that is lower than the firstcontent level of NO, and a second content level by volume of NO₂; (b)passing the adjusted gas stream through a particulate trap that resultsin trapping at least a portion of the particulate matter on theparticulate trap; (c) combusting a portion of the trapped particulatematter in the presence of the adjusted gas stream to reduce a combustiontemperature necessary to stop significant accumulation of particulatematter in the particulate trap relative to the combustion temperature ofa portion of the particulate matter in the presence of the exhaust gasnecessary to stop significant accumulation of particulate matter in theparticulate trap, and to create a further adjusted gas stream comprisinga third content level by volume of NO and a third content level byvolume of NO₂ that is lower than the second content level of NO₂; (d)injecting a reductant fluid comprising urea into the further adjustedgas stream; (e) mixing the further adjusted gas stream with the injectedreductant fluid to form a further adjusted gas stream mixed withreductant fluid; and (f) passing the further adjusted gas stream mixedwith reductant fluid over an SCR catalyst to provide a final adjustedgas stream comprising a fourth content level by volume of NO and afourth content level by volume of NO₂; wherein the second content levelof NO₂ is sufficiently higher than the first content level of NO₂ suchthat when a portion of the second content-level of NO₂ in the adjustedgas stream is consumed during the combustion of the at least a portionof the trapped particulate matter, the resulting third content level ofNO₂ is still sufficiently high for use with the SCR catalyst to providethe final adjusted gas stream where the total combined volume of thefourth content level of NO and the fourth content level of NO₂ is lowerthan the total combined volume of the first content level of NO and thefirst content level of NO₂, and the total combined volume of the fourthcontent level of NO and the fourth content level of NO₂ is lowerrelative to the respective total combined volume of NO and NO₂ in afinal exhaust stream that would result from carrying out steps (b)-(f)starting with the exhaust gas instead of the adjusted gas stream. 11.The method of claim 10, wherein the diesel engine is a vehicle engine.12. The method of claim 10, wherein the diesel engine is a heavy dutydiesel truck engine.
 13. The method of claim 10, wherein the dieselengine is a turbocharged heavy duty diesel truck engine.
 14. The methodof claim 13, further comprising cooling the further adjusted gas stream.15. The method of claim 14, wherein the further adjusted gas stream iscooled by air supplied by the turbocharger.
 16. The method of claim 10,wherein the oxidation catalyst converts less than all of the NO in theexhaust gas to NO₂.
 17. The method of claim 13, wherein the furtheradjusted gas stream mixed with reductant fluid is at least 225° C. whenpassed over the SCR catalyst, and the final adjusted gas stream has morethan 90% less NO_(x) content by volume than the exhaust gas.
 18. Themethod of claim 17, wherein the final gas stream has at least 67% lessparticulate matter content by volume than the exhaust gas.
 19. A methodcomprising: (a) passing an exhaust gas from a diesel engine over anoxidation catalyst to provide an adjusted gas stream, the exhaust gascomprising a first content level by volume of NO, a first content levelby volume of NO₂, and particulate matter, and the adjusted gas streamcomprising a second content level by volume of NO that is lower than thefirst content level of NO, and a second content level by volume of NO₂;(b) passing the adjusted gas stream through a particulate trap thatresults in trapping at least a portion of the particulate matter on theparticulate trap; (c) combusting a portion of the trapped particulatematter such that there is no significant accumulation of particulatematter in the particulate trap in the presence of the adjusted gasstream at a combustion temperature that is lower than the temperaturenecessary to combust the trapped particulate matter in the presence ofthe exhaust gas such that there is no significant accumulation ofparticulate matter in the particulate trap, to create a further adjustedgas stream comprising a third content level by volume of NO and a thirdcontent level by volume of NO₂ that is lower than the second contentlevel of NO₂; (d) injecting a reductant fluid comprising urea into thefurther adjusted gas stream; (e) mixing the further adjusted gas streamwith the injected reductant fluid to form a further adjusted gas streammixed with reductant fluid; and (f) passing the further adjusted gasstream mixed with reductant fluid over an SCR catalyst to provide afinal adjusted gas stream comprising a fourth content level by volume ofNO and a fourth content level by volume of NO₂; wherein the secondcontent level of NO₂ is sufficiently higher than the first content levelof NO₂ such that when a portion of the second content-level of NO₂ inthe adjusted gas stream is consumed during the combustion of the atleast a portion of the trapped particulate matter, the resulting thirdcontent level of NO₂ is still sufficiently high for use with the SCRcatalyst to provide the final adjusted gas stream where the totalcombined volume of the fourth content level of NO and the fourth contentlevel of NO₂ is lower than the total combined volume of the firstcontent level of NO and the first content level of NO₂, and the totalcombined volume of the fourth content level of NO and the fourth contentlevel of NO₂ is lower relative to the respective total combined volumeof NO and NO₂ in a final exhaust stream that would result from carryingout steps (b)-(f) starting with the exhaust gas instead of the adjustedgas stream; and wherein the further adjusted gas stream mixed withreductant fluid is at least 225° C. when passed over the SCR catalyst,and the final adjusted gas stream has more than 90% less NO_(x) contentby volume and at least 67% less particulate matter content by volumethan the exhaust gas.
 20. The method of claim 19, wherein the dieselengine is a vehicle engine.
 21. The method of claim 19, wherein thediesel engine is a heavy duty diesel truck engine.
 22. The method ofclaim 19, wherein the diesel engine is a turbocharged heavy duty dieseltruck engine.
 23. The method of claim 22, further comprising cooling thefurther adjusted gas stream.
 24. The method of claim 23, wherein thefurther adjusted gas stream is cooled by air supplied by theturbocharger.
 25. The method of claim 19, wherein the oxidation catalystconverts less than all of the NO in the exhaust gas to NO₂.